1. Field of the Invention
The present invention relates to surface acoustic wave (SAW) filter devices including a plurality of SAW elements, and in particular, to a SAW filter device in which a plurality of SAW resonators define parallel arm resonators and series arm resonators such that a ladder circuit is provided.
2. Description of the Related Art
Various types of SAW filters according to the prior art have been proposed in which a series arm SAW resonator is provided in a series arm between an input and an output of the SAW filter and a parallel arm SAW resonator is provided in a parallel arm between the series arm and the ground potential. SAW filters having a ladder circuit configuration have low loss and broad bandwidth. Accordingly, SAW filters are used as bandpass filters, and are widely used as cellular-phone bandpass filters.
According to Japanese Unexamined Patent Application Publication No. 5-183380, as shown in FIG. 29, the matching of a ladder type SAW filter is improved by connecting unit filters 501 and 502 as indicated by broken lines such that the configurations of the unit filters 501 and 502 are symmetrical. A unit filter is a circuit portion including a series arm resonator and a parallel arm resonator. In the SAW filter shown in FIG. 29, three unit filters 501 to 503 are connected. A combination of the resonators of the unit filters 501 and 502 is represented by the circuit diagram shown in FIG. 30.
In the circuit configuration shown in FIG. 30, series arm resonators S1 and S2 are connected between an input 504 and an output 505, and parallel arm resonators P1 and P2 are provided in two parallel arms. Where the capacitance of the parallel arm resonator P1 on the input side is represented by Cp1, and the capacitance of the parallel arm resonator P2 in the parallel arm located between the series arm resonators S1 and S2 is represented by Cp2, the ratio Cp2/Cp1=2 exists.
Japanese Unexamined Patent Application Publication No. 11-55067 discloses a structure in which the ratio Cp2/Cp1 is at least 2 and an inductor that is connected to an end of a parallel arm resonator P2 positioned between series arm resonators has a capacitance that is greater than that of an inductor connected to another parallel arm resonator. With this structure, attenuation in a frequency domain (frb to fra) lower than a pass band is achieved, as shown in FIG. 31.
It is common knowledge that, in a ladder SAW filter according to the prior art which includes a plurality of series arm resonators and a plurality of parallel arm resonators, the capacitances Cp1 and Cp2 of the parallel arm resonators may be set such that the ratio Cp2/Cp1=2 is obtained where impedance matching is important, and to ensure attenuation in a range lower than a pass band the ratio Cp2/Cp1 may be set to at least 1:2, and an inductor is connected to an end of a parallel arm resonator in a parallel arm located between the series arm resonators having a capacitance larger than that of an inductor connected to another parallel arm resonator.
A case in which the ratio Cp2/Cp1=2 as described in Japanese Unexamined Patent Application Publication No. 5-183380 has a problem in that the steepness of filter characteristics in a range lower than in the pass band deteriorates. The steepness of the filter characteristics in the lower range is indicated by a frequency distance xcex94f between attenuation values A to B in FIG. 32. The smaller the frequency distance xcex94f, the higher the selectivity.
As is disclosed in Japanese Unexamined Patent Application Publication No. 11-55067, where the ratio Cp2/Cp1 is at least 2 and an inductor that is connected to an end of a parallel arm resonator provided between series arm resonators has an inductance that is greater than that of an inductor connected to another parallel arm resonator, attenuation is enhanced in a range lower range than the pass band, as described above. Also, the steepness of the filter characteristics in the lower range of the pass band is enhanced, although this feature is not clearly described in Japanese Unexamined Patent Application Publication No. 11-55067.
However, it is found that the structure disclosed in Japanese Unexamined Patent Application Publication No. 11-55067 causes deterioration in attenuation in a range higher than the pass band.
To overcome the above-described problems, preferred embodiments of the present invention provide a ladder circuit type SAW filter device that has sufficient attenuation in each stop band of ranges higher and lower than a pass band and has superior steepness of filter characteristics in a low frequency domain in the vicinity of the pass band, and a communication apparatus including such a novel SAW filter.
According to a preferred embodiment of the present invention, a ladder circuit type surface acoustic wave filter device includes a piezoelectric substrate, a plurality of parallel arm resonators and a plurality of series arm resonators provided on the piezoelectric substrate which are respectively defined by surface acoustic wave resonators, and a plurality of inductors connected in series to the parallel arm resonators. The parallel arm resonators include a first parallel arm resonator connected to one of an input end and an output end, and a second parallel arm resonator connected to a junction between two series arm resonators. The first parallel arm resonator and the second parallel arm resonator have a relationship represented by the following expression:
Cp1xc3x972 less than Cp2
where Cp1 represents the capacitance of the first parallel arm resonator, Cp2 represents the capacitance of the second parallel arm resonator. The inductor connected to the second parallel arm resonator has an inductance that is substantially equal to or less than the inductance of the inductor connected to the first parallel arm resonator.
Preferably, the capacitance Cp2 of the second parallel arm resonator is in a range represented by the following expression:
Cp1xc3x972 less than Cp2 less than Cp1xc3x9710.
The resonance frequency of the second parallel arm resonator may be lower than the resonance frequency of the first parallel arm resonator.
The ladder circuit type surface acoustic wave filter device according to another preferred embodiment of the present invention further includes a package which contains surface acoustic wave elements in which the parallel arm resonators and the series arm resonators are provided on the piezoelectric substrate and a plurality of electrode pads connected to the series arm resonators or the parallel arm resonators, and bonding wires to connect the electrode pads to the series arm resonators or the parallel arm resonators. The length of the bonding wire connected to the second parallel arm resonator is substantially equal to or less than the length of the bonding wire connected to the first parallel arm resonator such that the inductor connected to the second parallel arm resonator has an inductance that is substantially equal to or less than the inductance of the inductor connected to the first parallel arm resonator.
The ladder circuit type surface acoustic wave filter device according to another preferred embodiment further includes a package which contains surface acoustic wave elements in which the parallel arm resonators and the series arm resonators are provided on the piezoelectric substrate and a plurality of electrode pads connected to the series arm resonators or the parallel arm resonators, and bonding wires to connect the electrode pads to the series arm resonators or the parallel arm resonators. One end of the second parallel arm resonator is electrically connected to at least two electrode pads among the electrode pads which are connected to the ground potential such that the inductor which is connected to the second parallel arm resonator has an inductance that is substantially equal to or less than the inductance of the inductor connected to the first parallel arm resonator.
According to another preferred embodiment of the present invention, a communication apparatus includes at least one ladder circuit type surface acoustic wave filter device in accordance with other preferred embodiments of the present invention, wherein the at least one ladder circuit type surface acoustic wave filter defines a bandpass filter of the communication apparatus.
According to various preferred embodiments of the present invention, in a ladder circuit type surface acoustic wave filter device including a piezoelectric substrate, a plurality of parallel arm resonators and a plurality of series arm resonators which are provided on the piezoelectric substrate and which are respectively defined by surface acoustic wave resonators, and a plurality of inductors which are respectively connected in series to the parallel arm resonators, the parallel arm resonators include a first parallel arm resonator connected to one of an input end and an output end, and a second parallel arm resonator connected to a junction between two series arm resonators. The first parallel arm resonator and the second parallel arm resonator have the relationship Cp1xc3x972 less than Cp2, that is, the capacitance ratio Cp2/Cp1 is greater than 2, where Cp1 represents the capacitance of the first parallel arm resonator, Cp2 represents the capacitance of the second parallel arm resonator. Thus, the steepness of the filter characteristics in the lower range of a pass band is greatly improved. Because the inductor which is connected to the second parallel arm resonator has an inductance that is substantially equal to or less than the inductance of the inductor connected to the first parallel arm resonator, a sufficient amount of attenuation outside of the pass band is produced. Therefore, outstanding filter characteristics that not only have superior steepness but also have sufficient attenuation in a stop band are obtained.
In particular, when Cp1xc3x972 less than Cp2 less than Cp1xc3x9710, not only the steepness of the filter characteristics and the attenuation in the stop band are improved, but also sufficient bandwidth is achieved.
When the resonance frequency of the second parallel arm resonator is lower than the resonance frequency of the first parallel arm resonator, reflection characteristics in the lower range of the pass band do not deteriorate. This effectively enhances the steepness of filter characteristics.
In the present invention, members defining the inductors connected in series to the parallel arm resonators are not particularly limited. By controlling the length of the bonding wire connecting each electrode pad in the package and each parallel arm resonator, the inductance of the inductor connected to the parallel arm resonator is controlled. Specifically, the length of the bonding wire connected to the second parallel arm resonator is substantially equal to or less than the length of the bonding wire connected to the first parallel arm resonator, wherein the inductor connected to the second parallel arm resonator has an inductance that is substantially equal to or less than the inductance of the inductor connected to the first parallel arm resonator. Similarly, by electrically connecting one end of the second parallel arm resonator to the electrode pads via the bonding wires, the inductor connected to the second parallel arm resonator has an inductance that is substantially equal to or less than the inductance of the inductor connected to the first parallel arm resonator.
In a communication apparatus of a preferred embodiment of the present invention, the SAW filter device of other preferred embodiments of the present invention is preferably used as a bandpass filter. Accordingly, filter characteristics in the bandpass filter are greatly improved, such that a communication apparatus having superior selectivity is provided.
Other features, elements, characteristics and advantages of the present invention will become more apparent from the detailed description of preferred embodiments below with reference to the attached drawings.